Exemplary embodiments of the invention relate to a friction stir processing tool having a shoulder implemented on a body, which has a friction surface for pressing against at least one workpiece, which is at least partially provided with a surface coating. Furthermore, the invention relates to a friction stir processing device provided with such a friction stir processing tool. Finally, the invention relates to a friction stir processing method, in particular a friction stir welding method, which can be carried out therewith.
The invention is in the field of friction stir processing and in particular friction stir welding. Friction stir welding (FSW) is increasingly used in aerospace technology, in rail traffic technology, entertainment electronics, and in automotive engineering. It is a simple, clean, and innovative joining method, which is distinguished by a high potential for automation and a high efficiency, whereby the production costs are decreased and the weight of structures produced therefrom is reduced.
Friction stir welding was first described in PCT document WO 93/19935. In this case, two workpieces to be welded to one another are brought into contact and held in this position. A welding pin or a pin-shaped projection of a corresponding tool is inserted into the connecting region of the workpieces with rotating movement until a shoulder arranged above the welding pin on the tool rests on the surface of the workpieces. In this case, friction heat is generated by the relative movement between tool and workpieces, so that adjacent material regions in the connecting region assume a plasticized state. The tool is moved forward along a connecting line of the workpieces while the rotating welding pin is in contact with the connecting region, so that the material located around the welding pin plasticizes and subsequently consolidates. Before the material hardens completely the welding pin is removed from the connecting region or the workpieces, respectively.
Materials, for example, metals, their alloys, metal material composites—so-called MMCs—or suitable plastic materials can be welded in this manner as a butt joint, lap joint, or T-joint connection. Spot connections can also be generated, wherein a forward movement of the rotation with the welding pin in contact with the connecting region or a translational relative movement between rotating welding pin and workpieces does not occur.
However, the technique of friction stir processing is also applied in the repair, processing, and finishing of workpieces. In this case, as described above, a pin-shaped projection is inserted into at least one workpiece with rotating movement (i.e., welding is performed in solid material), to modify the workpiece at least in the contact region of the welding pin. For repair purposes, the rotating welding pin is inserted into a crack of a workpiece, for example.
German patent document DE 10 2005 030 800 B4 discloses a friction stir processing tool for friction stir welding for the purpose of connecting a first workpiece and a second workpiece along a weld seam as a connecting line and a corresponding friction stir welding method. In this known method, the pin and the shoulder or a body of the tool bearing the shoulder are rotatable in relation to one another. In particular, the shoulder is fixed during the welding, so that it does not execute a rotational movement (rotational speed n=0). Less friction heat is thus generated in the region of the surface.
Many novel possible welding tasks, for example, the connection of workpieces of entirely differing material consistency, result by way of this welding method known from German patent document DE 10 2005 030 800 B4 and the corresponding tool. For example, metals having low melting temperatures can be welded to metals having higher welding temperatures. This is possible in that the friction heat can be monitored better and therefore the material that softens early can be kept in a plasticized and non-liquid state. Thus, for example, connections are even possible between aluminum workpieces and titanium workpieces. Mixed connections of aluminum-steel are also possible.
However, if workpieces having a surface coating are processed in the friction stir processing method, this leads, as a result of the relative movement between shoulder and workpiece, to the surface coating being damaged.
For example, using the friction stir processing tool described in German patent document DE 10 2005 030 800 B4 and the friction stir welding method described therein, welding can be performed between a galvanized steel plate and an aluminum plate. The friction of the shoulder during the advance of the friction stir processing tool over the connecting line damages the zinc layer of the steel plate in many cases.
Damage to the coating by the shoulder of the friction stir processing tool, for example, the welding tool, generally occurs if friction stir welding processes or friction stir processing processes in general are applied to coated joined parts or workpieces.
Heretofore, the coating of the joined parts or the workpiece was locally reapplied in the weld region after completed welding.
Exemplary embodiments of the invention are directed to friction stir processing methods on coated workpieces having simpler processes.
According to a first aspect, the invention provides a friction stir processing tool having a shoulder implemented on a body, which has a friction surface for pressing against at least one workpiece, which is at least partially provided with a surface coating, and a friction reducing unit for reducing the friction resistance between the surface coating and the friction surface.
By the reduction of the friction resistance on the friction surface region, damage to coatings of a workpiece to be processed may be decreased or avoided, so that a subsequent further local application of the coating can be avoided.
According to one preferred embodiment of the invention, a region of the shoulder that comes into contact with the surface coating of the material to be processed is implemented as more yielding than the surface coating.
It is preferable for the friction reducing unit to have at least one support element for supporting at least a partial region of the friction surface such that at least the partial region of the friction surface is more yielding than the surface coating of the friction surface.
The friction reducing unit is preferably implemented such that the friction forces or the friction load on one partial region of the friction surface of the shoulder are selectively kept lower than on another partial region of the friction surface of the shoulder.
According to a further aspect, the invention provides a friction stir welding tool having a multipart tool shoulder.
This is advantageous for friction stir welding of mixed connections in particular, in which one workpiece is provided with a sensitive surface coating and another workpiece is not.
For example, a first workpiece is formed from galvanized steel plate, which is to be welded onto a second workpiece made of aluminum.
There are many further possible examples of pairs of first and second workpieces, of which a first workpiece is provided with a surface coating.
It is preferable for the friction stir processing tool to be provided with a shoulder implemented on a body, which has a friction surface for pressing against a first workpiece and a second workpiece to be connected to one another.
It is preferable for the friction surface to comprise a first surface region and a second surface region, and the friction reducing unit to be implemented only on a partial region of the body such that the friction resistance on this first surface region of the friction surface is less than the friction resistance on the second surface region of the friction surface.
The first workpiece provided with the surface coating can be engaged with the first surface region, while the vertical process forces are transferable more via the second surface region.
The reduction of the friction can be performed in various ways. Provision with a friction-reducing layer is conceivable. However, the vertical contact pressure forces on the first surface region are preferably less than on the other surface region. This may be achieved by greater yielding of the first surface region than the second surface region.
It is preferable for the support element to be implemented to support the first surface region and to be softer or more yielding than a supporting surface of the second surface region.
It is preferable for the first surface region to be implemented on a region of the body formed from a plastic material and the second surface region to be formed on a region of the body formed from a metal and/or ceramic material.
According to one embodiment, it is preferable for the first surface region to be formed by a first half of the friction surface and the second surface region to be formed by the second half of the friction surface.
Such an embodiment is particularly advantageous for friction stir welding methods, in which a movement of the friction stir processing tool occurs centrally to the connecting line between the workpieces.
According to a further embodiment, it is preferable for one of the surface regions, preferably the first surface region, to be implemented on a partial segment of the friction surface which occupies less than half of the friction surface.
It is preferable for a partition line, which separates the first surface region from the second surface region, to extend off-center to the friction surface. The partition line is preferably arranged such that the second surface region extends by a predefined distance beyond a center line, which can be arranged in an advance direction for the friction stir processing tool, of the friction surface.
Embodiments having an off-center partition between surface regions are advantageous, for example, for friction stir welding methods in which the shoulder is applied and moved off-center to a connecting line between workpieces. This is advantageous, for example, during the welding of workpieces made of materials having significantly differing melting points. The material having the lower melting point plasticizes at lower temperatures than the material having the higher melting point. An off-center application and movement depending on the material composition can therefore be advisable; it is accordingly advantageous to adapt the distribution of the surface regions.
It is preferable for a pin, which protrudes from the shoulder and is drivable to rotate, to be provided, which is rotatable in relation to the shoulder.
It is preferable for the shoulder to be implemented, during a friction stir welding method for connecting a first workpiece and a second workpiece along a weld seam, to be pressed at a rotational speed n=0 against the surface of the first and/or the second workpiece.
According to a further aspect, the invention provides a friction stir processing device comprising a friction stir processing tool according to one of the above-explained embodiments and a tool guiding unit for guiding and moving the friction stir processing tool in an advance direction.
The friction stir processing device is particularly preferably a friction stir welding device, wherein the tool guiding unit is implemented for the purpose of guiding the friction stir processing tool along a connecting line between workpieces to be connected to one another to form a weld seam.
According to a further aspect, the invention provides a friction stir processing method for carrying out a friction stir process on at least one workpiece provided with a surface coating, comprising: using a friction stir processing tool according to one of the above-explained embodiments, wherein the shoulder is selected and/or implemented depending on the surface coating such that, when it is pressed against the surface coating and moved on the surface coating in an advance direction during the friction stir process, it does not damage or at least does not completely abrade or penetrate the coating.
It is preferable for the friction stir processing method to be a friction stir welding method for connecting a first workpiece and a second workpiece by means of friction stir processing along a weld seam, wherein the first workpiece is provided with a surface coating, and the friction stir processing method further comprises: engaging the surface coating using the first surface region and transferring axial forces by means of the second surface region.
A particularly preferred embodiment of the friction stir welding method is characterized by connecting a first workpiece, which is formed from galvanized steel, to a second workpiece, which is formed from a light metal, along the weld seam while pressing the first surface region onto the zinc layer of the first workpiece and pressing the second surface region onto the light metal of the second workpiece.
A further preferred embodiment of the friction stir welding method is characterized by using a friction stir processing tool having off-center partition line between first surface region and second surface region and carrying out the friction stir welding method using friction stir processing tool, which is to be moved offset in relation to the connecting line between first workpiece and second workpiece more toward the second workpiece in the advance direction, wherein the distance of the partition line from the center line is selected as a function of the materials to be connected such that the partition line may be kept in the vicinity of the weld seam. The partition line is preferably to be kept and the distance X is to be selected precisely at the transition between further surface-coated workpiece surface region and plasticized weld seam region.
Exemplary embodiments of the invention will be explained in greater detail hereafter on the basis of the appended drawings.